Thermionic emission model for contact resistance in organic field-effect transistor

Injection mechanism of top-contact pentacene field-effect transistor (OFET) was investigated in respect to the internal field. The contact resistance was evaluated by the transmission line method for various applied external voltages as well as various pentacene film thicknesses. The behaviour of contact resistance was described in terms of the thermionic emission model (Schottky injection) and internal electric field generated by excess charges accumulated on pentacene–gate insulator interface. It was shown that pentacene film thickness changes the internal electric field affecting the carrier injection barrier. It was concluded that the space-charge field effect made a significant contribution for smaller pentacene film thicknesses and therefore in accordance to the thermionic model was able to decrease contact resistance representing the potential drop.     

Critical bending radius and electrical behaviors of organic field effect transistors under elastoplastic bending strain

Critical bending radii of some organic thin films were predicted using nanoindentation data and a prediction equation. Electrical behaviors of elastically bent organic field effect transistor(OFET) were also calculated assuming that dimensional changes in the films give rise to electrical changes. The predicted critical bending radius of a pentacene film (4.42 mm) agreed well with the experimental radius from the literature (4.6 mm) within a difference of 4%. The relative source-drain current behaviors predicted for pentacene OFETs using a conventional prediction equation are quite different from the experimental behaviors reported in the literature.     

High-performance triisopropylsilylethynyl pentacene transistors via spin coating with a crystallization-assisting layer

The effects of spin speed and an amorphous fluoropolymer (CYTOP)-patterned substrate on the crystalline structures and device performance of triisopropylsilylethynyl pentacene (TIPS-PEN) organic field-effect transistors (OFETs) were investigated. The crystallinity of the TIPS-PEN film was enhanced by decreasing the spin speed, because slow evaporation of the solvent provided a sufficient time for the formation of thermodynamically stable crystalline structures. In addition, the adoption of a CYTOP-patterned substrate induced the three-dimensional (3D) growth of the TIPS-PEN crystals, because the patterned substrate confined the TIPS-PEN molecules and allowed sufficient time for the self-organization of TIPS-PEN. TIPS-PEN OFETs fabricated at a spin speed of 300 rpm with a CYTOP-patterned substrate showed a field-effect mobility of 0.131 cm(2) V(-1) s(-1), which is a remarkable improvement over previous spin-coated TIPS-PEN OFETs.     

Optical Properties of Solid Pentacene

We have prepared pentacene films on Si, Al₂O₃, and glass substrates by thermal evaporation and have investigated their optical properties at room temperature over a wide range of frequencies from infrared to ultraviolet (5 meV to 6.5 eV). A series of optical phonon modes and electronic transitions have been observed. The internal vibrational modes in the infrared region match well their molecular counterparts but the electronic transitions show substantial changes from those of a free pentacene molecule. The HOMO–LUMO gap energy of the pentacene films deposited on Al₂O₃ and glass substrates is 1.85 eV.     

Performance improvement of pentacene organic field-effect transistor through introducing polymer buffer layers

Pentacene organic field-effect transistors (OFETs) employing poly(methyl methacrylate) (PMMA), polystyrene and polyvinylidene fluoride as the electrode buffer layers by simple spray-coating fabrication process were systematically investigated. Significant performance enhancement of the OFETs was obtained. By analyzing the morphologies of pentacene films grown on gold electrodes and the electrical characteristics of these OFETs, the performance improvement was attributed to the uniform and hydrophobic properties of polymer surface, leading to a remarkable reduction of contact resistance at the pentacene/electrodes interface. Moreover, the results showed that the device employing PMMA as the electrode buffer layer exhibited the highest hole mobility of 0.59 cm²/Vs, which was almost five times of the control one. Such effect was ascribed to the optimal surface energy and appropriate dielectric constant of PMMA, which were favorable for the growth of pentacene crystal and responsible for the highest performance of OFET using PMMA as the electrode buffer layer.     

Optical effects of doped top layers in silicon-on-insulator structures formed by ion implantation

Arsenic ions were implanted into silicon-on-insulator (SOI) structures at an incident energy of 100 keV to a dose of 2 × 10¹⁵ cm^–2. Conductive top layers were formed in the SOI structures after annealing at 1200 °C for 20 s. Infrared reflection spectra in the wave number range of 1500–5000 cm^–1 were measured and interference fringes, related to free-carrier plasma effects, were observed. By detailed theoretical analysis and computer simulation of infrared reflection spectra, the carrier concentration, the carrier mobility, and the carrier activation efficiency were obtained. The physical interpretation of the results and a critical discussion of the sensitivity of the data, fitted to variation in the parameters, are given.     

Transient charge accumulation in pentacene field effect transistor with silver electrode

Time-resolved microscopic optical second harmonic generation (TRM-SHG) imaging was employed to study a transient charge accumulation in top-contact pentacene field effect transistor (FET) with Ag electrodes. It was demonstrated that the SHG signal at the edge of the Ag electrode decayed but remained in a steady state depending on biasing condition. An electric field formed in pentacene layer below Ag electrode activates the SHG, indicating the insufficient accumulation of injected carriers in the FET channel. By using the TRM-SHG technique transient change of the carrier density in the OFET is obtained.     

Optical second harmonic generation measurements for investigating electron injection into a pentacene field effect transistor with Au source and drain electrodes

The pentacene field effect transistors (FETs)' operation for the injection carrier was revealed by means of the drain current-elapsed time (I_ds-t) and optical second harmonic generation (SHG) measurements. The charge carriers forming the conducting channel of pentacene FETs were mainly holes injected from the Au source electrode. Carrier injection from source and drain electrodes was followed by the carrier trapping, and the SHG signal modulated by the change in the electric field distribution between Au the source and drain electrodes was shown. In particular, at the off state of the FET, electron injection and succeeding trapping were suggested. Furthermore, hole injection assisted by trapped electrons was also suggested.     

Stability and 2,4-dinitrotoluene response of organic field effect transistors based on π-conjugated thiophene oligomers

This paper reports on organic field effect transistors (OFETs) based on two π-conjugated oligomers derived from thiophenes and their use as sensors for the detection of 2,4-dinitrotoluene (DNT). The detection mechanism relies on donor-acceptor interactions between the π-conjugated system (donor) and the nitrated molecule (acceptor). An important feature of sensors is the stability under operation, so, a large part of this work will be dealing with the behavior of OFETs under bias stress experiments as well as with the influence of temperature during operation. Most of results reported here are concerning hexyl capped tetra Thienylene–Vinylene (denominated 4-TV). Some preliminary results on the promising hexyl capped quinquethiophene derived from 3,4-ethylenedioxythiophene (denominated TETET) are also reported. Under a DNT contaminated air atmosphere ( 7 ppm), 4-TV based OFETs exhibit an increase of the drain current when DNT is present in the atmosphere as expected.     

Impedance spectroscopy measurements of charge carrier mobility in 4,4'-N,N'-dicarbazole-biphenyl thin films doped with tris(2-phenylpyridine) iridium

The charge carrier mobility of green phosphorescent emissive layers, tris(2-phenylpyridine) iridium [Ir(ppy)₃]-doped 4,4'-N,N'-dicarbazole-biphenyl (CBP) thin films, has been determined using impedance spectroscopy (IS) measurements. The theoretical basis of mobility measurement by IS rests on a theory for single-injection space-charge limited current. The hole mobilities of the Ir(ppy)₃-doped CBP thin films were measured to be 10^− 10–10^− 8 cm²V^− 1 s^− 1 in the 2–7 wt.% Ir(ppy)₃-doped CBP from the frequency dependence of both conductance and capacitance. These hole mobility values are much lower than those of the undoped CBP thin films (~ 10^− 3 cm²V^− 1 s^− 1) because the Ir(ppy)₃ molecules act as trapping centers in the CBP host matrix. These mobility measurements in the Ir(ppy)₃-doped CBP thin films provide insight into the hole injection process.     

Substrate- and oxidation-induced roughness of individual terraces of pentacene thin films

The roughness at the surface of individual pentacene terraces on naturally oxidized silicon wafers was investigated with scanning force microscopy as function of film thickness (one to five layers) and sample exposure to ambient air. For pristine samples, the root-mean-square roughness on individual (001) pentacene terraces was 0.18 nm and varied by less than 0.02 nm between monolayer terraces and terraces in the fifth layer. Storing samples in air and ambient light led to a substantial increase of the roughness, which for terraces up to the third layer became 0.24 nm after four weeks. For fourth layer terraces, the roughness increased less, and terraces in the fifth layer exhibited no significant roughness increase. We explain the roughness increase by photo-oxidation of pentacene, particularly strong within the first layer, which is supported by the appearance of grain boundary widening with storage time. The observation that layers beyond the third one from the substrate are less affected by photo-oxidation (smaller terrace roughness) is likely due to better structural perfection in layers farther from the substrate, which reduces the effective cross-section of molecules for oxidation. These results indicate that native silicon oxide does not allow for the immediate formation of structurally perfect pentacene films in the range of one to three layers, which will reduce charge carrier mobility in pentacene thin film transistors. Thicker pentacene layers can protect underlying layers against oxidation.     

Stability of pentacene organic field effect transistors with a low-k polymer/high-k oxide two-layer gate dielectric

This paper reports on the processing and the characterization of pentacene organic field effect transistors (OFETs) with a two-layer gate dielectric consisting of a polymer (PMMA) on a high-k oxide (Ta₂O₅). This dielectric stack has been designed in view to combine low voltage operating devices, by the use of a high-k oxide which increases the charge in the accumulation channel and the gate capacitance, and highly stable devices which generally could be achieved with polymer dielectrics but not necessarily with strongly polar high-k oxides. Bi-layer dielectric devices were compared to those with only Ta₂O₅ or PMMA gate insulators. Bias stress at room temperature was used to assess the electrical stability. A very low operating voltage was achieved with Ta₂O₅ but these devices exhibit hysteresis and degraded characteristics upon bias stress. OFETs with PMMA revealed very stable but operate at rather a high voltage due to the low dielectric constant of PMMA. Reasonably stable devices operating at about 10 V could have been obtained with PMMA/Ta₂O₅ two-layer dielectric. The origin of observed threshold voltage shift and mobility decrease upon bias stress are discussed.     

Charge-carrier injection into pentacene thin film formed on Si(111) probed by STM spectroscopy

The injection of charge carriers into a pentacene thin film formed on a Si substrate was investigated by scanning tunneling microscopy (STM). Tip height versus bias voltage (z-V) spectroscopy reveals the characteristic charge transport properties of the molecular film, i.e., the conductivity and the threshold energy of charge injection. The abrupt descent of the tip into the film owing to the transition of film conductance, which depends on the degree of charge carrier injection, was observed for crystallized pentacene thin films. The lower film conductance at around zero bias voltage is still higher than that of a vacuum. This indicates that the carrier injection barrier between the pentacene and the semiconducting substrate is extremely low. The convergence of the carrier injection endpoints into a narrow range of electric-field intensity implies that the main factor contributing to barrier formation and collapse is not the bias voltage but the electric field.     

MOCVD of p-Cd x Hg1 – x Te/GaAs Heteroepitaxial Structures

Metalorganic chemical vapor deposition from Cd and Te alkyl compounds and Hg vapor is used to grow p-type Cd _x Hg_{1 – x} Te epitaxial layers on semi-insulating GaAs(111)Bsubstrates by the interdiffused multilayer process (alternating CdTe and HgTe layers) at a substrate temperature of 350°C, followed by postgrowth annealing. Layers are obtained with x = 0.2–0.4, 77-K carrier concentrations in the range (1–5) × 10¹⁶ cm^–3, and 77-K carrier mobilities from 200 to 400 cm²/(V s). The rocking curves of the epilayers have a full width at half maximum in the range 2–4 min of arc.     

Properties of GaN grown on sapphire substrates

Epitaxial growth of GaN on sapphire substrates using an open-tube growth furnace has been carried out to study the effects of substrate orientation and transfer gas upon the properties of the layers. It has been found that for the (0001) substrates, surface appearance was virtually independent of carrier gas and of doping levels. For the (1 ¯102) substrates surface faceting was greatly reduced when He was used as a transfer gas as opposed to H₂. Faceting was also reduced when the GaN was doped with Zn and the best surfaces for the (1 ¯102) substrates were obtained in a Zn-doped run using He as the transfer gas. The best sample in terms of electrical properties for the (1¯102) substrate had a mobility greater than 400 cm² V^–1 sec^–1 and a carrier concentration of about 2 × 10¹⁷ cm^–3. This sample was undoped and used He as the transfer gas. The best (0001) sample was also grown undoped with He as the transfer gas and had a mobility of 300cm²V^–1 sec^–1 and a carrier concentration of 1 × 10¹⁸ cm^–3.     

Pentacene nanostructural interlayer for the efficiency improvement of polymer solar cells

In poly(3-hexylthiophene) mixed with phenyl C61-butyric acid methyl ester heterojunction polymer solar cells, organic small molecular pentacene was introduced as the interfacial layer between PEDOT:PSS coated ITO substrates and polymer layer. It is found that the short circuit current density and power conversion efficiency were distinctly improved due to the introduction of the nanostructural pentacene interlayer. The nearly 100% power conversion efficiency improvement was obtained on the cells with a 4 nm pentacene interlayer, which benefits from the increased short circuit current from 2.34 mA/cm² to 5.76 mA/cm². The morphology of different thicknesses of pentacene thin films was observed by atomic force microscopy. The effect of pentacene interlayer's thickness on the distribution of light in the active layer was simulated by using a transfer matrix mode.     

Reduction of threshold voltage fluctuation for organic field effect transistors by increase of insulator capacitance

Fluctuation of threshold voltages for top contact organic field effect transistors with pentacene active layers were successfully reduced by increase of gate insulator capacitance. Average fluctuation of threshold voltages of devices with 1000-nm-SiO₂-insulator was approximately 4 V, however it became only 0.5 V when the SiO₂ thickness was reduced to 100 nm. Since a vast number of transistors with constant threshold voltage are needed for application to displays, increase of capacitance would be a useful method for improving the reliability of OFETs.     

The fast alternative cryogenic experiment testbed

Subsystems for a “proof of concept” cryogenic payload have been developed to demonstrate the ability to accommodate low temperature science investigations within the constraints of the Hitchhiker siderail (HH-S) carrier on the Space Shuttle. These subsystems include: a hybrid solid neon – superfluid helium cryostat, a multi-channel Versa Modular European (VME) architecture Germanium Resistance Thermometer (GRT) readout and heater control servo system, and a multiple thermal isolation stage “probe” for thermal control of helium samples. The analysis and tests of these subsystems have proven the feasibility of a cryogenic HH-S carrier payload.     

Impact of the interfacial traps on the charge accumulation in organic transistors

A silver nanoparticles (NPs) self-assembled monolayer (SAM) was introduced in the pentacene organic field-effect transistors using modified Langmuir–Schäffer (horizontal lifting) technique, and its effect was being evaluated electrically using current–voltage (I?V) and impedance spectroscopy (IS) measurements. The I?V results showed a significant negative threshold voltage shift indicating that hole trapping phenomena exist inside the devices with about two orders higher in the contact resistance and an order lower in the effective mobility when a SAM was introduced. The IS measurements with the simulation of Maxwell–Wagner equivalent circuit revealed that the incorporation of the NPs SAM had a comparable negative voltage shift as in I–V measurements with a higher trapping time based on the simulation results.     

Rectifying properties of poly(N-methylaniline)

The electrical properties of various metal (Al/In/Sb/Sn)/poly(N-methylaniline)/gold Schottky diodes are described. The temperature-dependent dark current (J)–voltage (V ) characteristics indicate that the junctions formed with Al/In possess better rectifying properties with Schottky emission. In the case of junctions formed with Sb/Sn, Poole–Frenkel emission appears to be the dominant mechanism. The junction parameters of ideality factor, reverse saturation current, and barrier potential have been evaluated and compared with the reported values for Schottky barriers based on polyaniline and a few of its substituted derivatives. The capacitance (C)–voltage (V ) data obtained at different frequencies have been used to evaluate the carrier concentration, mobility, depletion width, contact potential, and work function of the polymer.